1. Field of the Invention
The invention concerns a density reducing high carbon content steel or a UHC-steel (Ultra High Carbon) which contains, besides iron and conventional impurities, from 0.8 to 2.5% C, 3.5 to 15% Al, 0.5 to 5% Cr, 0.01 to 4% Si, and up to 4% Ni, Mn, Mo, Nb, Ta, V, and/or W, as well as additional alloy components 0.1 to 0.85 Sn and 0 to 3% Ti, Be and/or Ga. In particular, the invention concerns superplastic UHC-steels.
The term “superplasticity”, with regard to metals, is understood to mean the capacity to withstand degrees of deformation upon application of a very low yield stress, without lateral contraction and practically no work hardening, which compared to materials having normal plasticity of approximately 10 to 40%, is several hundred to over 1000% for superplastic materials. A fundamental characteristic of the superplastic behavior of materials is the strong dependence of the yield strength on the rate of elongation or, as the case may be, elongation rate ({acute over (ε)}).
Superplastic deformation occurs using time controlled diffusion processes, during which very fine and often also rounded crystallites flow and rotate past each other. Thus, only a very narrow process window of temperature and deformation speed (elongation rate) ({acute over (ε)}) is allowed, in order to achieve the elongation values of the superplastic deformation of several 100 to 1000%. Typically herein a higher deformation temperature, above approximately 50% of the melting temperature (in ° C.), and a very low deformation speed of approximately 10−2 to 10−5 s−1, can be mentioned as guide.
2. Description of the Related Art
In machine construction and in the automobile industry superplastic metals offer a high potential in order to produce components with a high degree of deformation. Superplastic alloys are known for example from FR 274 1360 Al, U.S. Pat. No. 5,672,315, EP 1 252 352 Al, or U.S. 2001 020 502.
From U.S. Pat. No. 5,445,685 UHC-steels with 0.5 to 2.1% carbon and the following additional essential alloy components are known:0.5 to 10% Al, 1 to 16% Cr and optionally 0.2 to 2% Mn0.5 to 10% Al, 0.25 to 5% Mo, 0.25 to 5% Cr and optionally 0.2 to 2% Mn0.5 to 10% Al, 0.25 to 5% Si, 1 to 7% Cr, and optionally 0.2 to 2% Mn0.5 to 10% Al, 0.25 to 5% Ni, 1 to 7% Cr, and optionally 0.2 to 2% Mn0.5 to 10% Al, 0.5 to 10% Mn, 0.5 to 7% Cr.For adjusting the superplastic characteristic a special controlled cooling is carried out, which leads to the formation of spheric carbides.
For the mass production of components of interest it is important to have, besides the very high maximal degree of deformation, likewise also a high speed of deformation. Since acceptable deformation speeds can be realized only at elevated temperatures, the scaling or oxidation of the alloys during the deformation process can lead to a substantial problem. This applies particularly for Ta/Al alloys, however also for steels.
In order to meet the requirements of light construction in the motor vehicle industry, steels with reduced density are of particular interest.